Conversion of natural gas to liquid hydrocarbons (“Gas To Liquids” or “GTL” process) is based on a 3 step procedure consisting of: 1) synthesis gas production; 2) synthesis gas conversion by FT synthesis; and 3) upgrading of FT products (wax and naphtha/distillates) to final products.
The Fischer-Tropsch reaction for conversion of synthesis gas, a mixture of CO and hydrogen, possibly also containing essentially inert components like CO2, nitrogen and methane, is commercially operated over catalysts containing the active metals Fe or Co. Iron catalysts are best suited for synthesis gas with low H2/CO ratios (<1.2), e.g. from coal or other heavy hydrocarbon feedstock, where this ratio is considerably lower than the consumption ratio of the FT-reaction (2.0-2.1). The present invention is concerned with Co-based catalysts, in particular, supported Co-based catalysts. A variety of products can be made by the FT-reaction, but from supported cobalt, the primary product is long-chain hydrocarbons that can be further upgraded to products like diesel fuel and petrochemical naphtha. Byproducts can include olefins and oxygenates.
To achieve sufficient catalytic activity, it is customary to disperse the Co on a catalyst carrier, often referred to as the support material. In this way, a larger portion of Co is exposed as surface atoms where the reaction can take place. The present invention is concerned with alumina, as a support material.
Supported cobalt catalysts are the preferred catalysts for the FT synthesis. The most important properties of a cobalt FT catalyst are the activity, the selectivity usually to C5+ and heavier products and the resistance towards deactivation. Known catalysts are typically based on titania, silica or alumina supports and various metals and metal oxides have been shown to be useful as promoters.
In a paper by Iglesia et al. [“Selectivity Control and Catalyst Design in the Fischer-Tropsch Synthesis: Sites, Pellets and Reactors” Advances in Catalysis, Vol 3, 1993] a Thieles modulus is defined as a product of two components, Ψn and χ, where Ψn depends only on the diffusivity and reactivity of the individual molecules, whereas χ depends only on the physical properties and site density of the catalyst. They have described a model whereby the selectivity to C5+ products can be described as a volcano plot in terms of χ. The structural parameter is given as:χ=Ro2Φθm/rp,where θm is the site density, e.g. as the number of surface atoms of Co metal atoms per cm2 of pore area in the catalyst particle, Ro is the diffusion length, i.e. the radius of an essentially spherical catalyst particle, Φ is the porosity of the particle (cm3 pore volume/cm3 particle volume) and rp is the mean pore radius.
This expression suggests that χ only depends on universal constant, characteristic data for cobalt in the catalyst as well as the size and density of the catalyst particles. It is particularly significant that χ does not depend on the pore radius, rp. However, it now appears that the selectivity of the Fischer-Tropsch reaction to C5+ products indeed does in fact depend on the pore size.
In a paper by Saib et al. [“Silica supported cobalt Fischer-Tropsch catalysts: effect of pore diameter of support” Catalysis Today 71 (2002) 395-402], the influence of the effect of the average pore diameter of a silica support on the properties of a cobalt catalyst and their performance in F-T synthesis is discussed. The article concludes that the support pore diameter has a strong effect on cobalt crystallite size with larger crystallites forming in larger pore sizes. Also, the activity was found to be a function of the metal dispersion and the maximum C5+ selectivity a function of the conversion.
In EP 1 129 776 A1 it is argued that internal diffusion phenomena in a catalyst particle depend on the chemical and morphological structure of the catalyst (pore dimensions, surface area, density of the active sites) and on the molecular dimensions of the species in question. This is a general teaching found in relevant textbooks, e.g. expressed in terms of the Thiele modulus, and it is significant that the pore dimension, i.e. the pore radius or diameter is one of the critical parameters. Further, it is taught that for the Fischer-Tropsch synthesis, interparticle diffusion will create low concentrations of CO towards the centre of the particle with a consequent progressive rise in the H2/CO ratio inside the catalyst and that this condition favours the formation of light hydrocarbons (lower a-value and C5+ fraction). On the other hand, it is stated that multiphase reactors of the slurry type generally use small catalyst particles (20-150 mm) which do not give internal diffusion problems, and more specifically that for catalysts based on differently supported cobalt used in the Fischer-Tropsch synthesis, it is possible to neglect internal diffusion limitations by operation with particles having diameter of less than 200 mm.
In EP 0 736 326 B1, it is shown that the C5+ selectivity can increase over a certain range of increasing pore size for a cobalt on alumina type FT catalyst. However, no reference or details of the method of measuring pore size is given, and it is well known that reported values vary significantly with method, e.g. for different probe gases or whether adsorption or desorption isotherms are employed.
In general, after impregnation of an alumina carrier with a solution of a cobalt catalyst material, the carrier is dried and calcined at a relatively low temperature of 200 to 450° C., e.g. at 300° C., for 2 to 16 hours. However, it is known that prolonged calcination at higher temperatures, e.g. above 500° C. can reduce catalyst activity.
This is in the first instance due to agglomeration of Co crystallites giving a reduced Co surface area for the FT-reaction, but at higher temperatures Co reacts with the alumina itself to form an inactive spinel phase, cobalt aluminate CoAl2O4. Transformation to cobalt aluminate was demonstrated by Davis and co-workers (Applied Catalysis, Volume 247, Pages 335-343, 2003) to occur at 650° C. and to transform completely at 850° C. The present invention relates to the surprising beneficial effect of high temperature treatment of an impregnated catalyst carrier on its attrition level.